1. Field
The present invention relates to treads for tires fitted to heavy goods vehicles and, more particularly, relates to the tread pattern of these treads and to the rubber compounds of which they are made.
2. Description of Related Art
Wet weather driving requires the most rapid possible removal of the water in the contact patch where each tire makes contact with the roadway so as to ensure that the material of which the tread is made makes contact with this roadway. To achieve this, grooves are formed on the tread which, through their dimensions (depth and width), remain open in the contact patch with the roadway and thus allowing water that is not pushed around the front and sides of the tire to be drained away.
The grooves may have any shape in cross section and in terms of the line they follow along the tread surface and may be orientated in any direction. The line followed by a groove on the tread surface here mean the mean geometric line followed by the edge corners formed by said groove on said surface.
For the tires intended for the steered or load-bearing axles of a heavy goods vehicle, it is usual practice to provide the tread of these tires with longitudinal grooves the depth of which is equal substantially to the total thickness of the tread (not including any thickness that may be provided in order to allow the grooves to be partially renewed). This depth as a general rule is comprised between 13 and 18 mm on these axles. For the tires intended for the driven axles, the groove depth may be as much as 24 mm.
For such tires of the prior art, the total voids volume content is, as a general rule, comprised between 15 and 25% of the volume of the tread intended to be worn away during driving. It is found that these tires have an available voids volume in the contact patch that is relatively large in the new state; this voids volume opening onto the tread surface in the contact patch is, on average, of the order of 100 cm3 for example for a tire of size 315/70 R 22.5. For the tire in question, this value is obtained for its nominal inflation and static load conditions as defined by the ETRTO standard.
Moreover, in order to improve the grip of the tire on the roadway, it is known practice to form sipes which generate edge corners on the tread surface. A sipe is a thin slit of small mean width and such that, under the usual loading conditions, the walls of material that delimit this sipe can at least partially come into contact with one another when passing through the contact patch where the tire is in contact with the road so as to limit the loss of stiffness. These sipes may be the depth equal to the thickness of the tread to be worn down or less than this thickness.
While grooves or more generally cavities are essential to draining water away, the resulting reduction in surface area may have appreciable effect on the wear performance of a tread and therefore reduce the service life of the tire as a result of an increase in wear rate. Other performance aspects of the tire may also be affected, notably performance regarding behaviour, road noise while driving, rolling resistance. It has also been found that these grooves formed to have a working depth equal to the height of tread to be worn away may cause endurance problems. Under certain driving conditions, foreign objects such as stones may become lodged in these grooves and attack the bottom of these grooves causing cracks to appear in the rubber.
Moreover, the grooves lead to a drop in the compression stiffness and shear stiffness because these grooves delimit portions of material which are more sensitive to deformation in comparison with the portions delimited by sipes. This is because, in the case of sipes, the walls of material delimiting these sipes can come into contact with one another at least when passing through the contact patch with the roadway. This reduction in stiffness, in the case of the presence of the grooves, leads to an increase in deformations and generates a reduction in tread wear performance: greater wear is noted for a set distance traveled (which corresponds to an increase in the tread wear rate). Moreover, an increase in rolling resistance and therefore in fuel consumption of vehicles fitted with such tires is also noted, this being the result of an increase in hysteresis losses associated with the deformation cycles of the material of which the tread is made.
The material from which to make the tread is generally chosen as a function of the use to which the tire is put: conventionally, the use of natural rubber makes it possible to obtain both a low level of hysteresis (therefore a low tire heating) and a very good resistance to attack and chunking. This is why materials based on natural rubber are usually employed in forming tire treads for heavy goods vehicles. The use of non-isoprene diene elastomers such as SBR copolymers of Tg higher than the Tg of natural rubber makes it possible to improve grip on wet surfaces. However, this is achieved at the expense of hysteresis and therefore of rolling resistance; this is why it is preferable to use functionalized SBRs which make it possible to maintain hysteresis levels compatible with the use in a tread for a heavy goods vehicle tire. However, the use of this type of elastomer, although accompanied by a marked improvement in wet grip, leads to a reduction in the resistance to chunking on impact, and notably when the impacts are to the lateral parts of the treads when driving up against obstacles.